1. Field of the Invention
The present invention relates to a measuring apparatus configured to measure a position of a test surface.
2. Description of the Related Art
Japanese Patent No. (“JP”) 4,554,385 proposes a white light interferometer configured to separate scanning of a test surface position into a wide-range measuring interval and a micro measuring interval, and to provide scanning with the micro measuring intervals for each wide-range measuring interval. JP 3,739,987 proposes an optical frequency comb interferometer configured to measure a test surface position without scanning a test or reference surface by utilizing an optical frequency comb for test and reference beams and by setting different frequency intervals.
However, the white light interferometer disclosed in JP 4,554,385 requires 105 image pickups or more for a measurement range of 100 mm even if the wide-range measuring interval is set to about several micro millimeters and thus is impractical due to enormous measurement times and a long measurement time period. The optical frequency comb interferometer disclosed in JP 3,739,987 utilizes two optical frequency comb sources and causes the measuring apparatus to be expensive. In addition, the measuring accuracy is limited due to the relative phase noises between these two optical light sources. Moreover, since the optical frequency comb interferometer disclosed in JP 3,739,987 condenses the test beam upon the test surface and measures the position of the test surface, scanning a spot position is necessary to measure the shape of the test surface and thus the measurement takes a long time.